1. Technical Field
The present invention relates to an apparatus for measuring a downward rotation amount of eyeball and a method for measuring a downward rotation amount of eyeball in a spectacle lens.
2. Related Art
Spectacle lenses include progressive power spectacle lenses in addition to single-vision spectacle lenses. The progressive power lens is an aspherical lens including a distance portion area having a refractive power (diopter) corresponding to distance vision and a near portion area having a refractive power corresponding to near vision. The distance portion area is set at an upper position of the lens, the near portion area is set at a lower position of the lens, and a progressive zone in which the refractive power progressively changes is provided between the areas. There is no border separating these areas, and a user can look at objects from far to near with one lens. The distance portion area, the near portion area, and the progressive zone need adjustments according to an individual's intended use (far and near focusing, intermediate and near focusing, near and near focusing, full-time use, part-time use, static use, dynamic use, etc.) (optical fittings). Among the optical fittings, a downward rotation amount of eyeball is important for the progressive power lens. The “downward rotation amount of eyeball” is, when a line-of-sight position on the lens in a state where a spectacles wearer views at eye level is defined as a distance-vision eye point (FP), and a line-of-sight position on the lens in a state of a line of sight for near vision is defined as a near-vision eye point (NP), a distance of the downward rotation of an eyeball from the distance-vision eye point to the near-vision eye point.
For designing such a spectacle lens, there is a related art in which, by analyzing an individual's eye movement path in a state of wearing spectacles using software with information from an eye movement measuring apparatus, one or more eye points or an average area is specified, and, based on the information, a spectacle lens is custom-designed by modifying a standard lens according to the individual's eye (JP-T-2008-521027). In the related art of JP-T-2008-521027, the eye movement measuring apparatus is used for measuring an eye point.
Moreover, there is a related art (JP-T-2003-523244) in which a head tracking system and values derived from the result of statistical analysis of a wearer behavioral statistical model are used, the wearer's individual visual behavioral patterns are determined, and a most suitable lens design is recommended with guidance on the choice of frame from a plurality of known lenses.
In the related art of JP-T-2008-521027, since the eye movement measuring apparatus is used, the apparatus is expensive as a whole. Since not only a head and an eye movement but also the posture of a person who wears spectacles (wearer) is involved in determining the distance-vision eye point or the near-vision eye point, the related art of JP-T-2008-521027 in which the points are determined only with the head and the eye movement cannot obtain a correct measured value.
In the related art of JP-T-2003-523244, similarly to the JP-T-2008-521027, since the head tracking system is used, not only is the apparatus expensive as a whole, but also the measurement cannot be accurately performed depending on the wearer's posture.